Substances showing the phenomenon of luminescence under the influence of X-rays are called phosphors.
According to a classical method of recording and reproducing X-ray images X-ray absorbing phosphor particles are used in phosphor panels or screens to form a radiographs in photographic silver halide emulsion materials sensitive to the fluorescent light promptly emitted by the phosphors where struck by X-rays.
In another radiographic method the principles of which are disclosed in U.S. Pat. No. 3,859,527 photostimulable phosphors are used. Said method comprises the exposure to X-rays of a photostimulable phosphor panel which phosphor stores at least part of the energy of the incident X-rays. At some interval after the exposure, a beam of visible or infra-red light scans the panel to stimulate the release of stored energy as light that is detected and converted into sequential electrical signals which are processable to produce a visible image.
According to the periodical Radiology, September 1983, p. 834 the imaging plate containing a photostimulable phosphor can be used repeatedly to store X-ray images simply by flooding it with light in order to erase the residual energy contained.
In the medical X-ray field it is often necessary to provide information on soft tissue structures and bone-like structures simultaneously. For that purpose "dual-energy" techniques operating in a single shot X-ray exposure with a low energy absorbing front screen and a high energy absorbing back screen separated by X-ray beam hardening filter have been used in classical screen-film assemblage exposure as well as in X-ray imaging with photostimulable storage phosphor screens (ref. Dev P. Chakraborty and Gary T. Barnes in Med. Phys. 16 (1), January/February 1989, p. 7).
The production of low- and high-energy images that are acquired simultaneously avoids image misregistration between the low- and high-energy exposures that could occur by patient-motion.
For the front screen preferentially absorbing the low-energy X-ray photons, useful in the imaging of the structure of soft tissue, low atomic number phosphors are used, whereas the higher energy part of the X-ray beam giving more information on the bone structure is absorbed or stopped in a back screen containing a higher atomic number phosphor.
In the above mentioned periodical Med. Phys. 16 (1), p. 7-8 an energy sensitive cassette for dual-energy mammography has been described wherein the back imaging plate contains a commercial barium fluorobromide phosphor and the front plate contains a similar but no commercially available strontium fluorobromide phosphor. The selection of these phosphors has been made in view of the K edges of Ba and Sr being respectively 37.4 and 16.1.
The K edge of Ba ensures high absorption efficiency of the barium fluorobromide phosphor above 38 keV, whereas the strontium fluorobromide is absorbing effectively in the X-ray region with energies from 15 to 25 keV.
In order to make that the lower energy X-ray radiation cannot interfere with the Ba-type phosphor an interscreen filter, also called midfilter in the form of a copper sheet is used (ref. Medical Physics, Vol. 17, No. 4, July/August 1990, p. 667).
The following phosphor combinations for classical screen-film dual energy X-ray recording were considered: SrFBr/BaFBr, Y.sub.2 O.sub.2 S/Gd.sub.2 O.sub.2 S and Y.sub.2 O.sub.2 S/LuTaO.sub.4.
In U.S. Pat. No. 5,051,589 a stimulable phosphor sheet suited for subtraction radiography is described wherein different stimulable phosphors are mixed together or take the form of a layer, wherein at least a first stimulable phosphor is represented by the formula GdOCl:Ce and a second stimulable phosphor is represented by the formula SrFCl:Eu.sup.2+. Said phosphors have different X-ray absorption characteristics which represent different absorptivity with respect to energy levels of X-ray radiation and have different response speeds with regard to stimulating rays which cause them to emit light in proportion to the amounts of X-ray energy stored.
In published European patent application 0 112 469 corresponding with U.S. Pat. No. 4,855,598 an X-ray pattern subtraction method is described comprising steps of:
(i) obtaining at least two X-ray images of an object including a specific structure exhibiting radiation energy absorption characteristics different from the radiation energy absorption characteristics of the other structure in said object, PA0 (ii) extracting an image of said specific structure by subtraction of one of said images from the other one, characterized by PA0 (iii) simultaneously exposing a plurality of stimulable phosphor sheets (A,B; A,B,B'; A,B,C) which are overlaid one upon another to provide a composite member, to the X-ray radiation passing through said object, to store radiation images in the respective stimulable phosphor sheets (A,B; A,B,B'; A,B,C) so that image information is recorded in a stimulable phosphor sheet (B; B,B'; B,C) positioned farther from said object in such a way that, at least at the area of the record corresponding to said specific structure, a low energy component of the radiation is recorded at a lower intensity than in a stimulable phosphor sheet (A) positioned closer to said object, PA0 (iv) scanning the respective stimulable phosphor sheets (A,B; A,B,B'; A,B,C) with stimulating rays to sequentially release the radiation energy stored in the stimulable phosphor sheets as light emission. PA0 (v) photoelectrically detecting the emitted light and converting it to digital image signals representing the respective X-ray radiation images, PA0 (vi) providing at least two sets of digital image signals, representing at least two respective X-ray radiation images, for use in subtraction processing, and PA0 (vii) extracting an image of said specific structure by subtraction processing of said sets of signals, i.e. multiplying digital image signals of corresponding picture elements by weight factors and subtracting said weighted signals.
For the use in image subtraction of weight factors for bones and soft tissue reference is also made to U.S. Pat. No. 5,049,748.
As described by John M. Boone, Melvin Tecotzky and Guillermo M. Alexander in the article "Binary Screen Detector System for Single-Pulse Dual-Energy Radiography" published in the periodical Radiology, June 1992; p. 863 the read out and digitization of images obtained from stacked detector screens during imagewise X-ray exposure proceeeds after separation of the stacked elements. During that separation and separate read out the spatial alignment between the images and detectors may be lost.
Published European patent application 0 126 564 and corresponding U.S. Pat. No. 4,752,557 relates to a radiation image storage panel and its preparation in which a photostimulable phosphor is provided on a substrate with a honeycomb structure constituted of a number of cells partitioned by wall members filled with the stimulable phosphor. According to said documents the radiation image storage panel can give images of markedly improved sharpness.
The use of said panel is separately recording a higher and lower energy level pattern from a single X-ray pattern has neither been disclosed nor suggested in said U.S. Pat. No. 4,752,557.